Absorbent products, such as diapers, training pants, napkins, incontinence pads and the like, contain an absorbent core usually comprising a soft, fluffy material, such as comminuted wood pulp fibers. Various secondary materials are often incorporated into the absorbent fluff material, such as particles of superabsorbent polymers (SAP), heat activatable bonding fibers or odour absorbent material. Super absorbent materials are polymers having the ability to absorb water and bodily fluids many times there own weight. The super absorbent material is either mixed with the fluff pulp fibers or applied in a layered configuration between layers of fluff pulp.
One way to produce such an absorbent core containing a second material is to form a first layer of absorbent fluff pulp, whereupon the second material is sprinkled on top of the fluff pulp. A second layer of absorbent fluff pulp is then placed on top of the second material to complete the core. The absorbent core may further comprise two or more layers of the second material disposed between layers of absorbent fluff pulp fibers. A result of this method is that it produces a product with a layered configuration, in which the second material is concentrated in fairly discrete zones within the core.
Alternatively, the second material (such as SAP particles) is mixed with and distributed throughout the first material (the absorbent fluff pulp fibers). It is further known to make absorbent products including a first and a second layer, in which the first layer contains a pure first material and the second layer contains a mixture of the first and the second material.
One way to produce an absorbent core for products with a fairly uniform distribution of the second material within the core is disclosed in patent document U.S. Pat. No. 5,447,677. All statements and drawings in said document are hereby included, by reference, in this description. In said disclosure there is presented an apparatus for making absorbent products containing a first material, such as absorbent fluff from wood pulp fibers. Said absorbent fluff pulp is introduced into a vacuum-forming chamber. A portion of the fluff pulp is deposited into the cavity of a mold transported through the forming chamber by a forming wheel so as to form a layer of pure fluff pulp within the bottom of the mold cavity. A second material, such as superabsorbent particles or heat activatable bonding fibers, is introduced into the forming chamber so that streams of the first and second materials collide within a mixing zone. As the mold continues it travels through the forming chamber. A mixture of first and second materials from the mixing zone is deposited within the mold cavity, thereby filling it. The result is an absorbent core having a first layer formed by pure first material and a second layer formed by a mixture of the first and second materials.
A more detailed description of a known manufacture of an absorbent product of the design as discussed is made here by reference to FIG. 1. From a defibration unit pulp fibers are transported by the aid of air up to hoods 1a, 1b and through the hoods towards a forming wheel 3. The forming wheel 3 is provided with stationary suction boxes 2, wherein very low pressure prevails. The suction boxes face said hoods containing the pulp fibers. On the surface of the forming wheel 3 an air pervious means, such as a net or a perforated plate, is provided. When said air pervious means rotates, in the direction shown by the arrow 3b, under said hoods the pulp fibers are forced by an air stream towards the air pervious means and form a core of pulp fibers along the circumference of the forming wheel in the shape of a continuous mat. Said circumference may be provided with molds for forming separate absorbent cores of different designs.
The core may further be formed into several layers by the use of several hoods. The core may, as stated, be formed into discrete cores or if a continuous mat-formed core is formed, it may be later cut into the desired design.
A flow of SAP-fibers is added to the pulp fibers by an injection of the SAP-fibers through a pipe 4 into one or more of the hoods 1. After the last hood (in the figure hood 1b) the core formation is completed and the core is kept in the mold or molds on the wheel by means of an underpressure in a second stationary suction box 5. The core is, after the passage past said second suction box 5, transferred to a transfer drum 5a, where it may be compressed, for example by means of a mechanical pressure exerted on the core during its passage over the transfer drum. Finally, the core is transferred to a conveyor 6 for further treatment or packaging. Transfer from the forming wheel 3 to the conveyor 6 may be to undertaken without the transfer drum.
An end product may consist of several cores, whereby the manufacturing assembly for said end product consists of two or more forming wheels 3, each one of the wheels forming a core, whereupon the two or more resulting cores are assembled to the end product.
A disadvantage in the discussed manufacturing process is that SAP-particles or fibers are easily lost from the core after the completion of the core formation on the forming wheel 3 along the distance the core travels until it is transferred to the conveyor. Even if a very high vacuum is utilized in the second suction box 5, it is very difficult to retain all of the SAP-particles in the formed core, especially if high speeds are used for the transporters, that is, the conveyor, the forming wheel and the transfer drum. Arrangements may be made to collect the lost SAP-particles in funnels. In this way, portions of the lost amount of SAP-particles are collected and may be returned to the core forming process. Other portions of the lost SAP-particles must be discarded. Still other portions of said particles land on the machinery, whereby the machinery has to be cleaned regularly. SAP-particles may also fall down on an underlying conveyor, which results in a product having SAP-particles in undesired locations of the core. Not only SAP-particles are lost during the transfer of the completed core to the conveyor, but also pulp fibers may escape during said transport of the core.
Another way to try to solve the indicated disadvantage would be to increase the vacuum inside the second suction box 5. This could cause the SAP-particles to penetrate deeper into the underlying first layer of pulp fibers and even to be sucked out of the core and clog the air pervious means or to escape into the second suction box 5 and cause problems in the vacuum generation equipment.